Despite the fact that prostate cancer (PC) is the second most common cancer among American men and the third leading cause of cancer mortality, relatively few epidemiologic studies of this disease have been conducted, and little is known about its etiology. A recent case- control study has suggested that vasectomy may be associated with as much as a two to four fold increase in the risk of PC. A case-control study of PC in relation to vasectomy among men under age 70 will be conducted in Massachusetts: 1128 men with recently diagnosed PC will be compared with twice as many age-matched neighbor controls. Information on vasectomy and potential confounding variables will be obtained by personal interview and a self administered food frequency questionnaire. Because few men chose vasectomy as a method of contraception prior to 1960, the first substantial cohort of vasectomized men is only now reaching the age of high PC incidence. This study will provide important information on an increasingly common and modifiable exposure, vasectomy, that may increase the incidence of PC. Secondary aims of this study are to evaluate the effect of other factors (confounders) such as sexual history, diet, prior drug use, other medical conditions, and occupation on PC risk. To assess the possibility of selection bias, cases will be analyzed separately according to clinical stage at diagnosis (disease limited to the prostate, disease extending beyond the prostate). Cases with locally extensive or metastatic disease are less susceptible to selection bias because symptoms will bring them to medical attention irrespective of prior medical history. Men with a prior history of PC or symptoms possibly due to undiagnosed PC (e.g., excessive nocturnal frequency, undiagnosed low back pain) will be excluded from the control series, virtually assuring a group free of advanced prostatic disease. Because of the high prevalence of PC in elderly men, it remains possible that some of men in the control series will have undiagnosed, asymptomatic PC; they will, however, be free of advanced, symptomatic tumors. To the extent that the control series includes men with small, undiagnosed PC lesions, a positive association of advanced PC with vasectomy will indicate that vasectomy is at least associated with an increased risk of advanced or aggressive PC but the effect of vasectomy on PC incidence may be underestimated.